This document claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2000-058301 filed on Mar. 3, 2000, the entire contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a laser printer, a digital copier, a laser facsimile, etc., and in particular, relates to a control and modulation device capable of controlling and modulating a light output of a semiconductor laser provided in the image forming apparatus.
2. Discussion of the Background
As background arts related to the present invention, Japanese patent application laid open nos. 05-075199, 05-235446, and 09-321376 describe technologies related to semiconductor laser beam (hereinafter simply referred to as laser) modulation. According to these publications, a light-electric negative feedback loop is formed so as to control a semiconductor laser at a high speed. Specifically, a prescribed current is induced by a light receiving element that monitors a light output of the semiconductor laser, and is always compared with a light generation instruction current. A prescribed amount of current is then carried to the semiconductor laser in proportion to that of the light generation instruction current in addition to an output current of the light-electric negative feedback loop. Thus, high speed modulation can be realized while suppressing adverse effects of temperature and droop characteristics of the semiconductor laser.
However, when a light output of a semiconductor laser is small, due to characteristics of the light receiving element, linearity of a light reception current output is remarkably deteriorated when compared with a light input. Thus, since control precision is inferior, a light is output in excess of a predetermined amount. As a result, an adverse effect appears in a laser printer or similar image forming apparatus (e.g., a dirty copy is made). In addition, since a control unit should normally be operated so as to always control a light output, a light output can not perfectly be turned OFF. As a result, an offset light generally arises. In addition, since a special circuit is needed in a laser printer or the like to set a predetermined amount of a driving current added to a semiconductor laser, a circuit scale of a light modulation IC should be limited when a function thereof is improved. In addition, when a semiconductor laser array or the like, which detects a plurality of outputs of laser beams with only one light receiving element, is utilized, an external device which separates and detects each of the light outputs should be provided.
A construction of one example of a background image forming apparatus is now described with reference to FIG. 20. As shown in FIG. 20, a laser beam is output from a semiconductor laser unit 134 and is deflected by a polygon mirror 135 when the polygon mirror 135 rotates. The laser beam scans a photo-conductive member (hereinafter referred to as a PC member) via an fxcex8 lens 136 and forms a latent image when the PC member 137 is exposed therewith. The semiconductor laser unit 134 controls formation of the latent image on the PC member 137 by controlling a light emission time period of the semiconductor laser in accordance with both of pixel data, which is generated by an image data processing unit 131, and a pixel clock having a prescribed phase, which is set by a phase synchronous circuit 139. In addition, the phase synchronous circuit 139 sets a prescribed phase to a clock generated by a clock generation circuit 132 so that the clock can synchronize with detection of a photo detector 138 that detects the laser beam deflected by the polygon mirror 135.
In general, the laser drive circuit 133, the phase synchronous circuit 139, and the clock generation circuit 132 are generally necessarily included in an image forming apparatus when a laser scan optical unit is employed and positional and interval precision of a latent image are obtained on the PC member 137.
However, since many clocks having the same frequency with a pixel clock are required in an image forming apparatus, a problem of electromagnetic interference generally arises.
In addition, since a number of parts increases, the image forming apparatus is costly. In addition, as a printing speed increases, a plurality of pixel clocks (as image data transfer tools) hardly operate at the same timing in an entire system, and data is generally transferred in parallel with a slowest clock.
Further, as a laser printer is operated at a higher speed and an image is printed with higher density, a system that includes a plurality of light sources is increasingly adopted. For example, a plurality of semiconductor lasers and a laser diode array are utilized as a light source. In addition, either one is preferably selected in consideration of an entire system. However, since a light receiving element is commonly utilized by all of semiconductor lasers when the LD array is utilized, a system proposed in the noted publications (i.e., Japanese patent application laid open nos. 05-075199, 05-235446, and 09-321376) can not be utilized, resulting in a high cost. In addition, as proposed in the publications (i.e., Japanese patent application laid open nos. 05-075199, 05-235446, and 09-321376), to avoid adverse effects of temperature and droop characteristics of a semiconductor laser, a prescribed control is continuously required.
However, an offset light concurrently arises with the continuous control. In addition, a circuit scale is large due to a current set circuit or similar devices. Also, a special device that separately detects each of light outputs is externally necessitated when a semiconductor laser array is utilized. Further, a beam profile of a semiconductor laser is generally similarly designed to a Gaussian distribution, and a latent image is formed in accordance with the Gaussian distribution in an electro-photographic system. Thus, the latent image does not appear in a binary state and a portion of analog like distribution arises when resolution is increased. This generally easily introduces influence of an external change such as a change in a developing bias, resulting in a change in density of an image.
For example, Japanese patent application laid open no. 11-167081 proposes a technology in which a frequency of a pixel clock can be set and changed by a direct synthesizer at a high speed. Specifically, frequency fractionation is changed by changing data of a look-up table (LUT). However, such changeable frequency fractionation and an output frequency changed speed generally give limitation on a total construction design due to close relation thereof to a phase locked loop control, which is described later in detail, and a low bandpass filter. In addition, since the frequency fractionation depends upon both of a master clock frequency and a number of bits of the LUT, it is necessary to increase a circuit scale or to increase a speed of the master clock. As a result, the above-described elements are typically not made into one chip.
In addition, a system proposed in Japanese patent application laid open no. 5-207234 in which a phase error is added to a PLL, generally produces a frequency error in a pixel clock unless a signal related to phase error addition is abnormally stable. This can also be an obstacle when both of digital and analog circuits are integrated on the one chip.
Accordingly, an object of the present invention is to address the above and other problems and provide a novel image forming apparatus.
The above and other objects are achieved according to the present invention by providing a novel image forming apparatus which includes a semiconductor laser that irradiates a laser beam modulated based on an image modulation signal, a reference clock generating device that generates a reference clock, and a pulse modulation device that generates a modulation pulse constituting the image modulation signal per one pixel from the reference clock and pixel data. In addition, the image modulation pulse includes at least a symmetrical thin pulse series.
In another embodiment, the image modulation pulse includes a thin center pulse between the symmetrical thin pulse series.
In yet another embodiment, a pixel clock generating device may be provided to generate a pixel clock that controls the laser beam. In addition, the pixel clock may include a plurality of pulses having a prescribed normal width constituted by a prescribed number of fractions (N), and substantially periodically include one or more pulses constituted by a greater width than the normal width. In addition, the greater width may be constituted by a number of fractions of (N) plus (M).
In yet another embodiment, a gradation error caused by a difference in fraction numbers (M) may be detected and diffused to one or more ambient pixels when the ambient pixels are digitized.
In yet another embodiment, the semiconductor laser is turned OFF during a time period corresponding to the fraction numbers (M) when the number (M) is positive.
In yet another embodiment, an intensity of the semiconductor laser may be increased during a time period corresponding to the fraction number (N) plus (M) when the number (M) is negative.